The resurgence of interest in the nuclear fuel cycle (NFC) is the result of the need to transition to a low or zero carbon emission economy. Uranium oxides are present at all stages of the NFC, and understanding their response to stimuli, especially temperature and pressure, and their tendency to form complex oxides incorporating other cations is important in developing stable long-term waste forms . This presentation is concerned with the structural chemistry of two Calcium Uranates, Ca3UO6 and Ca2UO5.
Ca3UO6 is a member of the double perovskite family, and is known to crystallising in the monoclinic space group P21/n. A second polymorph of Ca3UO6 , described as being metastable, is reported to form at relatively low temperature and this is denoted α-Ca3UO6. The accurate and precise structure of this polymorph has not been described. Preliminary synthetic and characterisation studies of this material by our group has confirmed this material crystallises in the rhombohedral R-3 space group. The structure is complex, consisting of both CaO6 and UO6 octahedra, where the CaO6 octahedra share edges with two neighbouring UO6 octahedra, and four edges with surrounding CaO6 octahedra. Thermographic analysis revealed a high temperature phase transition, and a thermal event at 600 °C reminiscent of oxygen loss. The thermal evolution of this material is in stark contrast to our previous work on the similar uranium-containing double perovskites such as Ba2SrUO6. Using Powder and Single-Crystal SXRD, the structure, of Ca3UO6 has been investigated in combination with the structure’s response to pressure and thermal stimuli.
Ca2UO5 has been reported to crystallise in the monoclinic space group P21/c. TGA analysis on this material revealed the presence of a thermal event at temperatures greater than 1000 °C and less than 1450 °C, however no noticeable events occurred at lower temperatures. A structural investigation of the thermal stability of this material identified an unexpected bond shortening above 600 °C. Further single crystal SXRD of Ca2UO5 at high pressures has also revealed a phase transition not reported before, from the monoclinic to a triclinic cell. These studies highlights the structural complexities of Ca2UO5 that are revealed through high resolution diffraction techniques.